This invention relates to improved waterfast inks for use in ink jet printing processes. More particularly, this invention relates to ink jet inks having excellent waterfastness and print quality characteristics that can be used in various printing processes such as thermal ink jet and acoustic ink jet processes.
Ink jet printing processes and apparatus for such processes are well known in the art. Two major types of ink jet processes are thermal ink jet and acoustic or piezoelectric ink jet processes.
In thermal ink jet printing processes, the printer typically employs a resistor element in a chamber provided with an opening for ink to enter from a plenum. The plenum is connected to a reservoir for storing the ink. A plurality of such resistor elements is generally arranged in a particular pattern, called a primitive, in a printhead. Each resistor element is associated with a nozzle in a nozzle plate, through which ink is expelled toward a print medium, such as paper. The entire assembly of printhead and reservoirs comprises an ink jet pen. In operation, each resistor element is connected via a conductive trace to a microprocessor, where current-carrying signals cause one or more selected elements to heat up. The heating creates a bubble of ink in the chamber, which is expelled through the nozzle toward the print medium. In this way, firing of a plurality of such resistor elements in a particular order in a given primitive forms alpha numeric characters, performs area-fill, and provides other print capabilities on the medium. The thermal ink jet printing process is described in more detail, for example, in U.S. Pat. Nos. 5,169,437 to You and 5,207,824 to Moffatt et al., the entire disclosures of which are incorporated herein by reference.
In an acoustic or piezoelectric ink jet system, ink droplets are propelled to the recording medium by means of a piezoelectric oscillator. In such a system, a recording signal is applied to a recording head containing the piezoelectric oscillator, causing droplets of the ink to be generated and subsequently expelled through the printhead in response to the recording signal to generate an image on the recording medium. In this printing system, a recording signal is converted into a pulse by a signal processing means such as a pulse converter and then applied to the piezoelectric oscillator. A change in pressure on the ink within an ink chamber in the printhead caused by the recording signal results in droplets of ink being ejected through an orifice to a recording medium. Such an ink jet system is described in more detail, for example, in U.S. Pat. No. 4,627,875 to Kobayashi et al., the entire disclosure of which is incorporated herein by reference.
In these and other ink jet recording processes, it is necessary that the ink being used meet various stringent performance characteristics. Such performance characteristics are generally more stringent than those for other liquid ink applications, such as for writing instruments (e.g., a fountain pen, felt pen, etc.). In particular, the following conditions are generally required for inks utilized in ink jet printing processes:
(1) the ink should possess liquid properties such as viscosity, surface tension and electric conductivity matching the discharging conditions of the printing apparatus, such as the driving voltage and driving frequency of a piezoelectric electric oscillator, the form and material of printhead orifices, the diameter of orifices, etc. PA1 (2) the ink should be capable of being stored for a long period of time without causing clogging of printhead orifices during use. PA1 (3) the recording liquid should be quickly fixable onto recording media, such as paper, film, etc., such that the outlines of the resulting ink dots are smooth and there is minimal blotting of the dotted ink. PA1 (4) the resultant ink image should be of high quality, such as having a clear color tone and high density. The ink image should also have high gloss and high color gamut. PA1 (5) the resultant ink image should exhibit excellent waterfastness (water resistance) and lightfastness (light resistance). PA1 (6) the ink should not chemically attack, corrode or erode surrounding materials such as the ink storage container, printhead components, orifices, etc. PA1 (7) the ink should not have an unpleasant odor and should not be toxic or inflammable. PA1 (8) the ink should exhibit low foaming and high pH stability characteristics.
Various inks for ink jet printing processes are known in the art. For example, various ink jet inks are disclosed in U.S. Pat. Nos. 4,737,190 to Shimada et al. and 5,156,675 to Breton et al. Generally, the ink jet inks of the prior art are aqueous inks, comprising a major amount of water, a humectant and/or a co-solvent, and a dye. By selecting specific humectants, dyes, or other components, it is possible to adjust the print characteristics of the resultant ink.
U.S. Pat. No. 5,364,462 to Crystal et al. describes dye-based inks that are described as providing improved stability, jetting characteristics, solubility and waterfastness. The aqueous dye-based ink includes a dye and a hydroxyethylated polyethylene imine polymer. The hydroxyethylated polyethylene imine polymer may also be substituted with hydroxypropylated polyethylene imine or epichlorohydrin-modified polyethylene imine polymers. Aprotic solvents, such as dimethyl sulfoxide and tetramethylene sulfone may also be added to the ink to improve the solubility and stability of the dye solution.
Sulfopolyester resins are known and are generally available commercially from Eastek Inks, a business unit of Eastman Chemical Company. These sulfopolyester resins are suitable for use in water-based inks, overprint lacquers and primers, as described in Kenneth R. Barton, "Sulfopolyesters: New Resins for Water-Based Inks, Overprint Lacquers and Primers," American Ink Maker, pp. 70-72 (October, 1993). The sulfopolyester resins may be prepared by the polycondensation reaction of selected dicarboxylic acids, glycols and sodio sulfoorganodicarboxylic acids or glycols to produce linear structures.
Although numerous ink jet inks are presently available, they generally do not meet all of the above-described requirements, while also providing excellent print quality on plain paper. In particular, the inks generally used in ink jet printing processes, while producing acceptable print quality, do not produce the high print quality that is achieved by using dry toner compositions, such as in electrostatographic imaging processes.
The need continues to exist in the ink jet industry for improved ink jet inks that satisfy the above-described requirements while providing high quality prints on a wide variety of recording media, including plain paper. Although some currently available ink jet inks may provide waterfast images with better substrate latitude, the inks are unacceptable in that they generally smear and have poor latency and maintainability characteristics. In addition, such inks are generally difficult to manufacture. Thus, there is still a need in the ink jet ink industry for improved black and colored inks that can be easily prepared and obtained at a lower cost.